Fluid valves of the rotary plug type are well known. The principal problem posed by such valves is the need to maintain a fluid seal between the rotating valve plug and the inlet and outlet ducts in the casing wall.
Failure to effectively seal these ducts, especially the inlet duct, not only prevents the valve from controlling the flow of fluid through the line, but frequently leads to erosion. If not remedied, eventual destruction of the plug, scouring of the inner wall of the casing, damage to the plug bearing, and fluid leakage are common.
The ability of a rotary plug valve to effectively seal its ducts is particularly important when the operating fluid is abrasive, chemically reactive, or extremely hot or cold.
U.S. Pat. No. 4,130,129 to Jack H. Pollack dicloses a rotary plug valve having a hydraulically augmented seal. A pair of sealing plugs are provided in the valve body to effectively seal the inlet and outlet ducts of the valve when under hydraulic pressure. However, the sealing plugs in the reference are under constant hydraulic pressure regardless of whether the valve is in an open or closed position. The constant pressure on the sealing plugs makes the valve difficult to open from a closed position and accelerates damage to the valve.
There is need for a rotary plug valve which effectively seals the inlet and outlet ducts but which does not have the disadvantages of the aforementioned U.S. Patent.